IRRIGATION AND SEEPAGE

Press, Volume LXXXII, Issue 24905, 19 June 1946, Page 9

IRRIGATION AND SEEPAGE

COMPARISONS WITH RAINFALL POSITION IN ASHBURTON DISTRICT An interesting comparison of the amounts of water deposited on the land by rainfall and by irrigation was given this week by Mr G. G. Calder, who is irrigation instructor for the Department of Agriculture at Ashburton. Mr Calder said that where seepage occurred on irrigated land it was likely that the farmer was putting too much water on, a wasteful proceeding, as not only did it waste water, but also it carried away some of the fertility of the soil.

“One can readily understand the trouble seepage is causing farmers; but irrigators should be, and are, concerned also, since, if irrigation is the cause of seepage, irrigation farmers must not only be irrigating too heavily but must be losing a considerable amount of fertility and plant food in the excess water that is supposed to be causing seepage from irrigation, said Mr Calder.

“Seepage is generally caused by rainfall or water Which falls on higher Jevels and seeps through the soil and sub-soil to emerge on lower levels as springs, etc. The flow of subsoil water continues for considerable distances underground. This usually follows along an impervious subsoil layer. It comes to the surface as springs if the impervious sub-soil layer also runs out to the surface of the land. Where light or shingly soil or subsoil meets heavy or clay subsoil, the subsoil water is forced to the surface. There may be other conditions that may block its passage to the sea. If one considers the Lyndhurst-Ashburton irrigation scheme in relation to irrigation water and rainfall some interesting figures are obtained.

170,000,000 Gallons a Month “The maximum .amount of irrigation water used this season was in February, when 7601 cusec hours, equal to 171 million gallons, was applied to the land. For March, which was more of an average month for irrigation, 3900 cusec hours were used, equivalent to 87 million gallons. This water was applied when the land was dry and losses from transpiration, and particularly evaporation, were high and When very little water would be lost through drainage to the subsoil. Experience has shown at Methven and Alexandra that losses from evaporation on a free water surface usually exceed the equivalent of the summer rainfall, and evaporation from a wet soil surface is greater than from a free water surface. “One inch of rain on the 68,000 acres of th£ irrigation scheme is equivalent to approximately 1525 million gallons of water. Thus for the heaviest irrigation, month, February, the total amount of irrigation water used was approximately 171 million gallons, compared with 1525 million gallons of a one inch rainfall, or by irrigation, the equivalent of approximately one ninth of an inch rainfall was applied to the land in the heaviest irrigating month.

Soil Conditions and Evaporation “The soil can absorb and hold one or two inches of water and possibly more without losing any in drainage when the soil is dry during the sum•mer,” continued Mr Calder. “However, when the soil is moist or wet, the water holding capacity is reduced and drainage increases. Thus if an inch or so of rain fell an the summer when evaporation was high it would be a refreshing rain but would provide no subsoil water and the land could be dry again in a short period and easily absorb another inch or so of rain or water. But an inch or so of rain in ,the autumn, winter, or spring, when the soil is wet and evaporation is low, will keep the soil Wet for a considerable time and probably supply some subspil water, while a second inch of rain within a short period would mostly be drained off as sfibsoil water. “Last year was a record-breaking one in many respects, record rainfall, record floods, record snow, and, as one would logically expect, a record seepage year also, and this naturally carried on into 1946. These records can hardly be connected with irrigation. Normally the summer period supplies very little if any subsoil water, but during, the abnormal 1945 year, February had more than 10 inches of rainfall and December more than five inches qf rainfall, both of which must have contributed considerably to the volume bf subsoil water, without taking into account the 5.87 inches and 5.80 inches which fell during May and August respectively. The record rainfall for 1945 of 42.59 inches was 44.9 per cent, more than the average, and must have some bearing in increased subsoil water which must ultimately appear somewhere as seepage or spring water.

Effects of Rain this Year “In 1946, the April rainfall of 2.42 inches has put the soil in a fairly moist condition, while the May rainfall of will provide a considerable amount of subsoil moisture. The May rainfall of 5.37 inches is approximately 11,239 million gallons over the irrigation scheme only, and appears rather significant when compared with 171 million gallons used on the same area for irrigation during February. While the figure of 171 million gallons of irrigation water is the total used for the month, the 11,239 million gallons of rainfall is only the amount which fell on the irrigated area, but if the total amount was calculated that t fell on the Ashburton County or that portion of the county between the Rakaia and Ashburtoh rivers, then the figure of 11,239 million gallons must be very largely increased.”